Update DavidAnson/markdownlint-cli2-action action to v21 (#247)

| datasource  | package                             | from    | to      |
| ----------- | ----------------------------------- | ------- | ------- |
| github-tags | DavidAnson/markdownlint-cli2-action | v20.0.0 | v21.0.0 |

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/lint.yml

@@ -51,7 +51,7 @@ jobs:
         uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
 
       - name: Lint Markdown
-        uses: DavidAnson/markdownlint-cli2-action@992badcdf24e3b8eb7e87ff9287fe931bcb00c6e # v20.0.0
+        uses: DavidAnson/markdownlint-cli2-action@30a0e04f1870d58f8d717450cc6134995f993c63 # v21.0.0
         with:
           config: .markdownlint-cli2.yaml
           globs: "**/*.md"